This invention is directed to a freely oscillating switched-mode power supply. Switched-mode power supplies having control only on the primary side of the isolating transformer have the advantage that there is no need for a transmission element for transmitting a controlled variable from the secondary side of the primary side of the transformer. However, there is the disadvantage that the voltages to be stabilized on the secondary side are not directly evaluated and thus the control for stabilizing these voltages is frequently not sufficient. Switched-mode power supplies having control on the secondary side to the primary side have the advantage that effective stabilization is achieved by the direct evaluation of the secondary-side voltages to be stabilized. However, there is the disadvantage that an element, such as an isolating transformer or an optocoupler, is required to transmit a controlled variable from the secondary side to the primary side.
When such switched-mode power supplies in a television receiver are switched over for active standby operation to a low transmitted power of approximately 5-8 watts, the switching transistor would need to be switched on only for very short times. This can result in relatively high switching losses and risk to the switching transistor. It is therefore known in the case of operation with low transmitted power to switch on the switching transistor only in a packet-like or burst-like fashion with a sufficiently long turn-on time and sufficiently high current and to cause it to be blocked therebetween for a relatively long time during which absolutely no power is transmitted. Switched-mode power supplies which operate according to this principle are relatively complicated in design and generally require special integrated circuits.